1. Field of the Invention
The present invention relates to a method for controlling and optimizing a xylene isomer separation and isomerization process using a near infrared analyzer system, and more particularly to a method and an apparatus for controlling and optimizing a xylene isomer separation and isomerization process, in which near infrared light of a wavelength ranging from 1,100 nm to 2,500 nm is transmitted through samples obtained at different stages of the process from raw materials flowing in the process by use of an analyzer system using optical fibers, thereby simultaneously measuring, in an on-line manner, xylene isomers and aromatic hydrocarbons containing 6 to 9 carbon atoms from those samples.
2. Description of the Prior Art
Referring to FIG. 1, a typical xylene isomer separation and isomerization process is illustrated. As shown in FIG. 1, such a xylene isomer separation and isomerization process mainly involves a xylene separation unit process, para-xylene separation unit process, and a xylene isomerization unit process, which processes are denoted by the reference numerals 1, 2, and 3, respectively, in FIG. 1. The raw material, which is processed in the xylene separation unit process, contains a large part of xylene isomer, ethylbenzene, toluene, and aromatic components. The xylene isomer is a mixture of meta-xylene, ortho-xylene, and para-xylene. In the xylene separation unit process, C9 aromatic components and ortho-xylene (partially) are separated from the raw material. The resultant material is then sent to the para-xylene separation unit process. In this para-xylene separation unit process, para-xylene is separated from the material. Thus, production of para-xylene is achieved. The resultant material containing xylene isomers other than para-xylene is then sent to the xylene isomerization unit process. Reactions carried out in the xylene isomerization unit process include an isomerization, in which the xylene isomers other than para-xylene are converted to have an equilibrium concentration, and a dealkylation, in which ethylbenzene is converted into benzene. The resultant material is then separated into a benzene product and an xylene isomer. The separated xylene isomer is subsequently circulated to the xylene separation unit process.
Meanwhile, in such a xylene isomer separation and isomerization process, several on-line and off-line gas chromatography devices have been used for analysis of process operations and process performances. Gas chromatography requires lengthened analysis time from 15 minutes to about one hour and a requirement for the use of a plurality of gas chromatography devices corresponding to respective stages where a required measurement is to be carried out. For these reasons, on-line gas chromatography devices have been used only to continuously check a few essential components associated with an optimum process operation, thereby obtaining products with a good quality.
In order to achieve a more accurate control and an optimization for processes, it is essentially required to calculate process performances of all unit processes used and the entire process. For such a process performance calculation, it is also required to analyze all components of respective materials to be processed in unit processes and respective product streams emerging from those unit processes. Typically, such a component analysis is carried out by obtaining samples in the production field, and measuring required components from those samples in the laboratory using an off-line gas chromatography device. However, such procedures require a lot of time. For this reason, in the case of the above mentioned xylene isomer separation and isomerization process, the process performance analysis is carried out only one time in a week. Based on the analysis results, process parameters are controlled. The process performance of the xylene isomer separation and isomerization process varies frequently depending on various variations occurring at the upstream stage from each unit process where a raw material to be processed is supplied. However, conventional process performance monitoring and process parameter control, which are carried out based on a component analysis using on-line and off-line gas chromatography devices, can not effectively cope with such variations.
On the other hand, U.S. Pat. No. 5,470,482 discloses a process control method in which control of fluidizing beds is carried out based on the purity or degree of recovery of para-xylene. In accordance with this method, the contents of para-xylene, ortho-xylene, meta-xylene, and ethylbenzene in pumparound and pusharound streams in a fluidizing beds are measured. Based on the measurement results, the purity or degree of recovery of para-xylene is calculated. The entire process is controlled in accordance with the relationships of the calculated purity or degree of recovery of para-xylene with operation parameters. This method is characterized in that control of fluidizing beds can be rapidly and efficiently achieved by measuring information required for separation control using an infrared and near infrared spectroscopy. However, this patent only shows the concept of measurement without any concrete implementing examples.
Meanwhile, Korean Patent Application No. 94-15408 discloses a process control method in which control of xylene-free fluidizing beds is carried out using a Raman spectroscopy. In accordance with this method, the chemical composition of a mixture of aromatic hydrocarbon isomers containing 8 to 10 carbon atoms is measured using a Raman spectrum. Based on the measurement result, the concentration profile of isomers in the mixture is reconstructed. The reconstructed concentration profile is then compared with a reference concentration profile determined by one or more operation parameters of a processing and controlling means used in the separation process. This method is characterized in that a distillation or crystallization process for obtaining para-xylene or ortho-xylene is carried out by controlling a simulated fluidizing bed. This method is basically different from the above mentioned method using near infrared because it uses a Raman spectroscopy to control xylene-free fluidizing beds.
After repeated experiments, the inventors could find that when particular stages in the xylene isomer separation and isomerization process are measured using a near infrared analyzer system, it is possible to solve the difficulties in process performance monitoring and process parameter control encountered upon using the above mentioned conventional component analysis based on a gas chromatography.